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ELECTRONIC DEVICES AND CIRCUITS

Faculty:1.Shaik.Jakeer Hussain2.P.Sandeep patil3.P.Ramesh Babu

UNIT-I

• ELECTRON DYNAMICS AND CRO: Motion of charged particles in electric and magnetic fields. Simple problems involving electric and magnetic fields only. Electrostatic and magnetic focusing. Principles of CRT, deflection sensitivity (Electrostatic and magnetic deflection), parallel and perpendicular electric and magnetic fields

Deflection of Electrons in a Uniform Electric Field

• Consider an electron beam directed between two oppositely charged parallel plates as shown below.

• With a constant potential difference between the two deflecting plates, the trace is curved towards the positive plate.

+

-

d

Deflection of Electrons in a Uniform Electric Field

• The force acting on each electron in the field is given by

d

eVeEF P

where E = electric field strength, V = p.d. between plates, d = plate spacing.

p

Deflection of Electrons in a Uniform Electric Field

• The vertical displacement y is given by

22 )(2

1

2

1t

md

eVaty p

2

2

)(2

1

v

x

md

eVp

This is the equation for a parabola.

Deflection of Electrons in a Uniform Magnetic Field

• The force F acting on an electron in a uniform magnetic field is given by

BevF

Since the magnetic force F is at right angles to the velocity direction, the electron moves rounda circular path.

Deflection of Electrons in a Uniform Magnetic Field

• The centripetal acceleration of the electrons is

m

Beva

Hence m

Bev

r

va

2

which gives

eB

mvr

Cathode Ray Oscilloscope (CRO)

• The structure of the cathode ray tube

Cathode Ray Oscilloscope Controls

• Y-Gain

• Time Base

Y-Gain

• amplifies the Y-deflection

• small input voltages are amplified by built-in amplifiers before applying to the Y-plates.

• Y- Gain = 0.5 V/div– 0.5 volt will cause a vertical deflection of 1

division

Time Base

• is a saw-tooth voltage applied internally across the X-plates.

time

volts

Time Base

• controls the speed at which the spot sweeps across the screen horizontally from left to right.

Time taken for spot to move across the screen and back

Fly backvolts

time0spot at centreof screen

spot on left sideof screen

spot on right sideof screen

volts

time0

Screen

spot on left sideof screen

spot at centreof screen

spot on right sideof screen

Fly back

Time Base

• it helps to display the actual waveform of any a.c. applied across the Y-plates

• normally calibrated in– s/cm– ms/cm– s/cm• gives the time required for the spot to sweep 1 cm

horizontally across the screen.

Time Base: How It Works

Time taken for spot to move across the screen and back

time

volts Fly back

0spot at centreof screen

spot on left sideof screen

spot on right sideof screen

A

B

C

Uses of c.r.o.

• Measure potential difference– d.c.– a.c.

• Display waveforms of alternating p.d.• Measure short intervals of time, and• Compare frequencies

Measuring d.c. Potential Difference

• switch off the time-base• a spot will be seen on the c.r.o. screen• d.c. to be measured is applied to the Y-

plates• spot will either deflected upwards or

downwards• deflection of the spot is proportional to the

d.c. voltage applied

Measuring d.c. Potential Difference

Y-input

y

If the Y-gain control is set at 2 volts/division

And the vertical deflection, y, is 1.5

Then d.c. voltage

= 1.5 x 2

= 3.0 V

Measuring a.c. voltage

• switch off the time-base• a spot will be seen on the c.r.o. screen• a.c. to be measured is applied to the Y-plates• spot will move up and down along the vertical axis at

the same frequency as the alternating voltage– spot moves to the top when the voltage increases to its

maximum (positive)– spot moves to the bottom when the voltage decreases to

its lowest (negative)

Measuring a.c. voltage

• When the frequency is high– the spot will move so fast that a vertical line is

seen on the screen• Length of the vertical line gives the peak-to-

peak voltage (Vpp) applied to the Y-plate• The peak voltage (Vp) is

= Vpp/2

Measuring a.c. voltage

Y-input

Vpp

Measuring a.c. voltage

Vpp

Vp Vp = Vpp/2

C.R.O. as a Voltmeter

• it has nearly infinite resistance (between the X- and Y-plates), therefore draws very little current;

• it can be used to measure both d.c. and a.c. voltages; and

• it has an immediate response.

Displaying Waveforms

• Set the time-base to a suitable frequency,• Apply the input to the Y-plate– a steady waveform of the input will be displayed

on the c.r.o.

Displaying Waveforms

Y-input

Displaying Waveforms

• When input voltage frequency is the same as the time-base frequency

Input Voltage c.r.o. screen

Displaying Waveforms

• When input voltage frequency is the twice the time-base frequency

Input Voltage c.r.o. screen

Measuring Short Time Intervals

• Set time-base to its lowest frequency range• Connect microphone to the Y-input• Blow two short whistles into the microphone– two short pulses, at short interval apart will be

displayed on the c.r.o. screen

Measuring Short Time Intervals

t divisions

If the time-base is 10 ms/division

and

if the separation between pulses

is t divisions

then

time interval is 10t ms

c.r.o. screen

Measuring Short Time Intervals

Y-input

Lissajous’ Figures

• Lissajous’ figure can be displayed by applying two a.c. signals simultaneously to the X-plates and Y-plates of an oscilloscope.

• As the frequency, amplitude and phase difference are altered, different patterns are seen on the screen of the CRO.

Lissajous’ Figures

Same amplitude but different frequencies